Ana Vader, Allegheny College

Class Year:

2023

Mentor:

Judy O'Neil, Ph.D. Jeffrey Cornwell, Ph.D.

Abstract:

The Susquehanna Flats, a fresh water site located near the mouth of the Susquehanna River, supports the greatest and most diverse population of submersed aquatic vegetation (SAV) in the Chesapeake Bay. The SAV bed traps suspended solids and removes nutrients before they approach downstream reaches of the Bay. A filamentous benthic cyanobacteria (Lyngbya) was first observed in the early 2000s and continues to reappear on a seasonal basis. The cyanobacteria grows on benthic and plant surfaces and as nutrients are depleted throughout the summer, it moves up the water column and begins fixing atmospheric nitrogen, eventually floating on the surface in dense mats. This research investigated potential negative interactions between Lyngbya and SAV by studying the effect of various nutrients on plant growth and nitrogen fixation rates as well as aerobic and anaerobic microbial dynamics within Lyngbya filaments and sediment through measuring oxygen and methane fluxes. Bioassays were conducted in triplicate with nutrient additions of 10 µM NO₃^-, 5 µM PO₄^3-, 10 µM NH₄⁺, and NO₃^- + PO₄^3- (N+P). Sub-samples from the bioassays were taken to measure the nitrogen fixation rates of the Lyngbya in the respective treatments using the acetylene reduction assay. Lyngbya that had sediments rinsed off (clean) and Lyngbya with co-occurring sediment left undisturbed (dirty) were compared for nitrogen fixation rates and nutrient exchange. Methanogenesis rates were analyzed for the sediment collected at Sea Grant Site 23. The bioassays demonstrated increased growth in July compared to June, with strong stimulation from the N+P treatment. Nitrogen fixation rates peaked with the addition of PO₄^3- in June with higher nitrogen fixation rates associated with Lyngbya that had sediment trapped in its matrix. Methanogenesis rates appeared to be higher in July in anaerobic conditions, correlating with decreased daily net oxygen production for Lyngbya samples with sediment. We hypothesize that when nutrients are high, the cyanobacteria can focus their energy towards growing and as nutrients become depleted, phosphorus in particular is drawn out of the sediment. Sediment nutrient uptake causes the Lyngbya to blanket the bottom substrate, changing the environment from aerobic to anaerobic. This research may contribute to Chesapeake Bay nutrient models of the SAV bed at the Susquehanna Flats.